
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>css 实现双边框的demo</title>
  <style>
  *{
    margin:0;padding:0;
  }
    .box {
      width: 100px; height: 50px; margin: 5px 2px 3px; padding:
       3px 2px 1px; border: 3px solid #000;
    }

  </style>
</head>
<body>
  <div class="box">双边框元素</div>
</body>
</html>
